The present invention relates to a printer, digital copier, facsimile apparatus or similar image forming apparatus for forming an image with optical writing means implemented as an LED (Light Emitting Diode) array head.
In an image forming apparatus for forming images by electrophotography, laser optics including a laser and a polygonal mirror for steering a laser beam issuing from the laser has been predominant over the other optical writing means. Today, an LED array head including an LED array and a lens array is attracting increasing attention as optical writing means capable of promoting the miniature and simple configuration of an image forming apparatus. The LED array has a number of LEDs arranged in an array and has the individual LEDs controlled in accordance with image data so as to electrostatically form a latent image on a photoconductive element.
It is, in practice, impossible to produce an LED array with LEDs all having an identical characteristic. As a result, dots to be formed by the LEDs are different in diameter from each other. Particularly, in a printer of the type rendering tonality with a bidimensional or area tonality method in which one dot has two levels, the difference in dot diameter directly translates into an irregular density distribution and thereby lowers image quality as to tonality. Further, the quantity of light to issue differs from one LED to another LED, also resulting in an irregular density distribution and therefore low image quality.
In light of the above, Japanese Patent Laid-Open Publication Nos. 5-4376 and 5-50653, for example, each discloses an image forming apparatus using an LED array head with an implementation for making the quantities of light to issue from LEDs uniform. In the former document, the LED array head is subjected to laser trimming so as to adjust resistance and therefore the quantity of light. In the latter document, correction data for a uniform quantity of light are determined and stored in a ROM (Read Only Memory) beforehand. At the time of printing, the correction data are used to selectively turn on the LEDs, so that all the LEDs emit the same quantity of light.
However, although all the LEDs may emit the same quantity of light, it is impractical to make the diameters of all the dots formed on a photoconductive element uniform due to, e.g., differences in dot diameter and focus between the LEDs. Moreover, even if correction is so effected as to make the dot diameters uniform, the actual dot diameters differ from each other, depending on the resolution of the correction data. As a result, vertical stripes appear in an output image. While the correction accuracy will somewhat increase if the resolution of the correction data is increased, such a scheme not only involves a limit, but also increases the cost of the apparatus.